Continuous casting of metals such as aluminum alloys is conventionally performed in twin roll casters, block casters and belt casters. Twin roll casting of aluminum alloys has enjoyed good success and commercial application despite the relatively low production rates achievable to date. Twin roll casting traditionally is a combined solidification and deformation technique involving feeding molten metal into the bite between a pair of counter-rotating cooled rolls wherein solidification is initiated when the molten metal contacts the rolls. Solidified metal forms as a “freeze front” of the molten metal within the roll bite and solid metal advances towards the nip, the point of minimum clearance between the rolls. The solid metal passes through the nip as a solid sheet. The solid sheet is deformed by the rolls (hot rolled) and exits the rolls.
Aluminum alloys have successfully been roll cast into ¼ inch thick sheet at about 4-6 feet per minute or about 50-70 pounds per hour per inch of cast width (lbs/hr/in). Attempts to increase the speed of roll casting typically fail due to centerline segregation. Although it is generally accepted that reduced gauge sheet (e.g., less than about ¼ inch thick) potentially could be produced more quickly than higher gauge sheet in a roll caster, the ability to roll cast aluminum at rates significantly above about 70 lbs/hr/in has been elusive.
Typical operation of a twin roll caster at thin gauges is described in U.S. Pat. No. 5,518,064 (incorporated herein by reference) and depicted in FIGS. 1 and 2. A molten metal holding chamber H is connected to a feed tip T which distributes molten metal M between water-cooled twin rolls R1 and R2 rotating in the direction of the arrows A1 and A2, respectively. The rolls R1 and R2 have respective smooth surfaces U1 and U2; any roughness thereon is an artifact of the roll grinding technique employed during their manufacture. The centerlines of the rolls R1 and R2 are in a vertical or generally vertical plane L (e.g., up to about 15° from vertical) such that the cast strip S forms in a generally horizontal path. Other versions of this method produce strip in a vertically upward direction. The width of the cast strip S is determined by the width of the tip T. The plane L passes through a region of minimum clearance between the rolls R1 and R2 referred to as the roll nip N. A solidification region exists between the solid cast strip S and the molten metal M and includes a mixed liquid-solid phase region X. A freeze front F is defined between the region X and the cast strip S as a line of complete solidification.
In conventional roll casting of aluminum alloys, the heat of the molten metal M is transferred to the rolls R1 and R2 such that the location of the freeze front F is maintained upstream of the nip N. In this manner, the molten metal M solidifies at a thickness greater than the dimension of the nip N. The solid cast strip S is deformed by the rolls R1 and R2 to achieve the final strip thickness. Hot rolling of the solidified strip between the rolls R1 and R2 according to conventional roll casting produces unique properties in the strip characteristic of roll cast aluminum alloy strip. In particular, a central zone through the thickness of the strip becomes enriched in eutectic forming elements (eutectic formers) in the alloy such as Fe, Si, Ni, Zn and the like and depleted in peritectic forming elements (Ti, Cr, V and Zr). This enrichment of eutectic formers (i.e., alloying elements other than Ti, Cr, V and Zr) in the central zone occurs because that portion of the strip S corresponds to a region of the freeze front F where solidification occurs last and is known as “centerline segregation”. Extensive centerline segregation in the as-cast strip is a factor that restricts the speed of conventional roll casters. The as-cast strip also shows signs of working by the rolls. Grains which form during solidification of the metal upstream of the nip become flattened by the rolls. Therefore, roll cast aluminum includes grains elongated at an angle to the direction of rolling.
The roll gap at the nip N may be reduced in order to produce thinner gauge strip S. However, as the roll gap is reduced, the roll separating force generated by the solid metal between the rolls R1 and R2 increases. The amount of roll separating force is affected by the location of the freeze front F in relation to the roll nip N. As the roll gap is reduced, the percentage reduction of the metal sheet is increased, and the roll separating force increases. At some point, the relative positions of the rolls R1 and R2 to achieve the desired roll gap cannot overcome the roll separating force, and the minimum gauge thickness has been reached for that position of the freeze front F.
The roll separating force may be reduced by increasing the speed of the rolls in order to move the freeze front F downstream toward the nip N. When the freeze front is moved downstream (towards the nip N), the roll gap may be reduced. This movement of the freeze front F decreases the ratio between the thickness of the strip at the initial point of solidification and the roll gap at the nip N, thus decreasing the roll separating force as proportionally less solidified metal is being compressed and hot rolled. In this manner, as the position of the freeze front F moves toward the nip N, a proportionally greater amount of metal is solidified and then hot rolled at thinner gauges. According to conventional practice, roll casting of thin gauge strip is accomplished by first roll casting a relatively high gauge strip, decreasing the gauge until a maximum roll separating force is reached, advancing the freeze front to lower the roll separating force (by increasing the roll speed) and further decreasing the gauge until the maximum roll separating force is again reached, and repeating the process of advancing the freeze front and decreasing the gauge in an iterative manner until the desired thin gauge is achieved. For example, a 10 millimeter strip S may be rolled and the thickness may be reduced until the roll separating force becomes excessive (e.g., at 6 millimeters), necessitating a roll speed increase.
This process of increasing the roll speed can only be practiced until the freeze front F reaches a predetermined downstream position. Conventional practice dictates that the freeze front F not progress forward into the roll nip N to ensure that solid strip is rolled at the nip N. It has been generally accepted that rolling of a solid strip at the nip N is needed to prevent failure of the cast metal strip S being hot rolled and to provide sufficient tensile strength in the exiting strip S to withstand the pulling force of a downstream winder, pinch rolls or the like. Consequently, the roll separating force of a conventionally operated twin roll caster in which a solid strip of aluminum alloy is hot rolled at the nip N is on the order of several tons per inch of width. Although some reduction in gauge is possible, operation at such high roll separating forces to ensure deformation of the strip at the nip N makes further reduction of the strip gauge very difficult. The speed of a roll caster is restricted by the need to maintain the freeze front F upstream of the nip N and prevent centerline segregation. Hence, the roll casting speed for aluminum alloys has been relatively low.
Some reduction in roll separating force to obtain acceptable microstructure in aluminum alloys having high alloying element content is described in U.S. Pat. No. 6,193,818, incorporated herein by reference. Alloys having 0.5 to 13 wt. % Si are roll cast into strip about 0.05 to 0.2 inch thick at roll separating forces of about 5,000 to 40,000 lbs/in at speeds of about 5 to 9 feet per minute. While this represents an advance in roll separating force reduction, these forces still pose significant process challenges. Moreover, the productivity remains compromised and strip produced according to the '818 patent apparently exhibits some centerline segregation and grain elongation as shown in FIG. 3 thereof.
A major impediment to high-speed roll casting is the difficulty in achieving uniform heat transfer from the molten metal to the smooth surfaces U1 and U2, i.e., cooling of the molten metal. In actuality, the surfaces U1 and U2 include various imperfections which alter the heat transfer properties of the rolls. At high rolling speeds, such non-uniformity in heat transfer becomes problematic. For example, areas of the surfaces U1 and U2 with proper heat transfer will cool the molten metal M at the desired location upstream of the nip N whereas areas with insufficient heat transfer properties will allow a portion of the molten metal to advance beyond the desired location and create non-uniformity in the cast strip.
Thin gauge steel strip has been successfully roll cast in vertical casters at high speeds (up to about 400 feet per minute) and low roll separating forces. The rolls of a vertical roll caster are positioned side by side so that the strip forms in a downward direction. In this vertical orientation, molten steel is delivered to the bite between the rolls to form a pool of molten steel. The upper surface of the pool of molten steel is often protected from the atmosphere by means of an inert gas.
While vertical twin roll casting from a pool of molten metal is successful for steel, vertical casting of alloys sensitive to oxidation (e.g., aluminum) requires additional control. One suggestion for overcoming this problem of oxidized aluminum in vertical roll casting on a laboratory scale is described in Haga et al., “High Speed Roll Caster for Aluminum Alloy Strip”, Proceedings of ICAA-6, Aluminum Alloys, Vol. 1, pp. 327-332 (1998). According to that method, a stream of molten aluminum alloy is ejected from a gas-pressurized nozzle directly onto one or both of the twin rolls in a vertical roll caster. Although high speed casting of aluminum alloy strip is reported, a major drawback to this technique is that the delivery rate of the molten aluminum alloy must be carefully controlled to ensure uniformity in the cast strip. When a single stream is ejected onto a roll, that stream is solidified into the strip. If a stream is ejected onto each roll, each stream becomes one half of the thickness of the cast strip. In both cases, any variation in the gas pressure or delivery rate of the molten aluminum alloy results in non-uniformity in the cast strip. The control parameters for this type of aluminum alloy roll casting are not practical on a commercial scale.
Continuous casting of aluminum alloys has been achieved on belt casters at rates of about 20-25 feet per minute at about ¾ inch (19 mm) gauge reaching a productivity level of about 1400 pounds per hour per inch of width. In conventional belt casting as described in U.S. Pat. No. 4,002,197, incorporated herein by reference, molten metal is fed into a casting region between opposed portions of a pair of revolving flexible metal belts. Each of the two flexible casting belts revolves in a path defined by upstream rollers located at one end of the casting region and downstream rollers located at the other end of the casting region. In this manner, the casting belts converge directly opposite each other around the upstream rollers to form an entrance to the casting region in the nip between the upstream rollers. The molten metal is fed directly into the nip. The molten metal is confined between the moving belts and is solidified as it is carried along. Heat liberated by the solidifying metal is withdrawn through the portions of the two belts which are adjacent to the metal being cast. This heat is withdrawn by cooling the reverse surfaces of the belts by means of rapidly moving substantially continuous films of water flowing against and communicating with these reverse surfaces.
The operating parameters for belt casting are significantly different from those for roll casting. In particular, there is no intentional hot rolling of the strip. Solidification of the metal is completed in a distance of about 12-15 inches (30-38 mm) downstream of the nip for a thickness of ¾ inch. The belts are exposed to high temperatures when contacted by molten metal on one surface and are cooled by water on the inner surface. This may lead to distortion of the belts. The tension in the belt must be adjusted to account for expansion or contraction of the belt due to temperature fluctuations in order to achieve consistent surface quality of the strip. Casting of aluminum alloys on a belt caster has been used to date mainly for products having minimal surface quality requirements or for products which are subsequently painted.
The problem of thermal instability of the belts is avoided in block casters. Block casters include a plurality of chilling blocks mounted adjacent to each other on a pair of opposing tracks. Each set of chilling blocks rotates in the opposite direction to form a casting region therebetween into which molten metal is delivered. The chilling blocks act as heat sinks as the heat of the molten metal transfers thereto. Solidification of the metal is complete about 12-15 inches downstream of the entrance to the casting region at a thickness of ¾ inch. The heat transferred to the chilling blocks is removed during the return loop. Unlike belts, the chilling blocks are not functionally distorted by the heat transfer. However, block casters require precise dimensional control to prevent gaps between the blocks which cause non-uniformity and defects in the cast strip.
This concept of transferring the heat of the molten metal to a casting surface has been employed in certain modified belt casters as described in U.S. Pat. Nos. 5,515,908 and 5,564,491, both incorporated herein by reference. In a heat sink belt caster, molten metal is delivered to the belts (the casting surface) upstream of the nip with solidification initiating prior to the nip and continued heat transfer from the metal to the belts downstream of the nip. In this system, molten metal is supplied to the belts along the curve of the upstream rollers so that the metal is substantially solidified by the time it reaches the nip between the upstream rollers. The heat of the molten metal and the cast strip is transferred to the belts within the casting region (including downstream of the nip). The heat is then removed from the belts while the belts are out of contact with either of the molten metal or the cast strip. In this manner, the portions of the belts within the casting region (in contact with the molten metal and cast strip) are not subjected to large variations in temperature as occurs in conventional belt casters. The thickness of the strip can be limited by the heat capacity of the belts between which casting takes place. Production rates of 2400 lbs/hr/in for 0.08-0.1 inch (2-2.5 mm) strip have been achieved.
However, problems associated with the belts used in conventional belt casting remain. In particular, uniformity of the cast strip depends on the stability of (i.e., tension in) the belts. For any belt caster, conventional or heat sink type, contact of hot molten metal with the belts and the heat transfer from the solidifying metal to the belts creates instability in the belts. Further, belts need to be changed at regular intervals which disrupts production.
Strip material of non-ferrous alloys are desirable for use as sheet product in the automotive and aerospace industries and in the production of can bodies and can end and tab stock. Conventional manufacturing of can body stock employs batch processes which include an extensive sequence of separate steps. When an ingot is needed for further processing, it is first scalped, heat treated to homogenize the alloy, cooled and rolled while still hot in a number of passes, hot finish rolled, and finally coiled, air cooled and stored. The coil may be annealed in a batch step. The coiled sheet stock is then further reduced to final gauge by cold rolling using unwinders, rewinders and single and/or tandem rolling mills. These batch processes typically used in the aluminum industry require many different material handling operations to move ingots and coils between what are typically separate processing steps.
Efforts to streamline production of can body stock are described in U.S. Pat. No. 4,260,419 via direct chill casting and U.S. Pat. No. 4,282,044 via minimill continuous strip casting. Both processes require many material handling operations to move ingots and coils. Such operations are labor intensive, consume energy and frequently result in product damage.
U.S. Pat. Nos. 5,772,802 and 5,772,799, incorporated herein by reference, disclose belt casting methods in which can or lid stock and a method for its manufacture in which a low alloy content aluminum alloy is strip cast to form a hot strip cast feedstock, the hot feedstock is rapidly quenched to prevent substantial precipitation, annealed and quenched rapidly to prevent substantial precipitation of alloying elements and then cold rolled. This process has been successful despite the relatively low production rates achievable to date.
In addition, alloys other than aluminum such as magnesium alloys have not been continuously cast on a commercial scale. Magnesium metal has a hexagonal crystal structure that severely restricts the amount of deformation that can be applied, particularly at low temperatures. Production of wrought magnesium alloy products is therefore normally carried out by hot working in the range of 300°-500° C. Even under those conditions, a multitude of rolling passes and intermediate anneals are needed. In the conventional ingot method, a total of up to 25 rolling passes with intermediate anneals are used to make a finished product of 0.5 mm gauge. As a result, magnesium wrought products tend to be expensive.
Accordingly, a need remains for a cost-effective method of casting of non-ferrous alloys that achieves uniformity in the cast surface.